U.S. Pat. Nos. 3,015,243; 3,115,052; 3,214,951; and 3,793,866 disclose machines for forming teeth in a workpiece by reciprocal movement of a pair of die racks that are driven along rectilinear paths. Hydraulic cylinders are utilized in the first three mentioned patents to move the die racks in a reciprocal manner along the rectilinear paths on opposite sides of the workpiece on which the teeth are being formed. One problem with conventional hydraulic cylinders used with machines of this type is that a different amount of fluid must be supplied to opposite sides of the cylinder piston during the extending and retracting strokes of the piston connecting rod of the cylinder. A greater amount of fluid must be supplied to the cylinder during extension than during retraction due to the larger area of the piston on the side which is not connected to the rod of the piston. Consequently, fluid pumped with equal pressure to the cylinder requires a greater time period to execute an extending stroke than a retracting stroke. This naturally makes the complete machine cycle time greater than twice the time of a retracting stroke. Also, pump apparatus for operating the machine cylinders is necessarily more complex due to the requirement of supplying a greater amount of fluid to the cylinder during one direction of movement than the other direction of movement.
Reciprocally driven cylinders are disclosed by U.S. Pat. Nos. 2,193,736; 2,624,318; 2,649,842; 2,664,859; 3,200,596; 3,592,108; 3,710,689; 3,744,375; and 3,949,650; and by British Patent specification No. 1,197,976.
In the machine disclosed by the U.S. Pat. No. 3,793,866 patent mentioned above hydraulic fluid rotary motors are used to drive the racks in opposite directions along rectilinear paths to provide tooth forming. Upper and lower base portions on which the racks are slidably mounted are interconnected by a connecting portion of the machine base and by a pair of rods whose opposite ends are threaded to nuts on the upper and lower base portions. As the hydraulic fluid rotary motors drive the die racks to form teeth in a workpiece, the rods limit the degree of deflection permitted between the upper and lower base portions in cooperation with the connecting portion of the base.